No building exceeding 2 1/2 stories or 36 feet in height shall be erected on any lot, except that, if a building shall be built on a slope, that portion of the building or structure on the lowest natural grade of the slope may be 48 feet in height, including all roof appurtenances of every kind and nature (notwithstanding the definition of building height), and provided that the roof at that portion of the building structure at the lowest grade does not stand any higher than the height of the roof at that portion of the building or structure at the highest grade. All roofs shall be free of appurtenances, towers, equipment, stacks, shaft extensions and the like and shall have no accessory structures thereon other than pumping vents, cupolas, stair and elevator towers, fresh air supply and exhaust, all properly screened, it being the intention of this provision to prevent obstruction of view, light and air.